Silver alloys has been used for the manufacture of articles such as decorative jewelry and tableware, as well as other formed, extruded and molded pieces for many years. The main element in these alloys is silver, a precious metal desirable for its bright white color tone. However, use of pure silver alone is not feasible due to its lack of inherent hardness.
Alloys of silver have traditionally been used to improve hardness and other qualities. The use of copper (Cu) is known for improving the hardness of silver. Other additional elements can be useful in imparting desirable qualities to silver metal. Advantages to the use of various silver alloys include their amenability to being cast from molten metal, malleability, and strength or hardness.
The presence of copper in a silver alloy can be problematic as copper has a propensity to form discoloring sulfides and chlorides. Additionally, silver-copper alloys often oxidize to form a black- or red-colored blemish commonly known as “fire scale”. It is believed that fire scale is copper oxide. The cupric variety will produce a blackened blemish, whereas the cuprous variety will form a reddish blemish. In either case, this blemish is not limited to the surface of the article, as in the case of superficial tarnish (typically silver sulfide), but may penetrate the article more deeply. In some cases, the penetration is such that the defect or blemish cannot be removed by buffing and polishing.
Consequently, a disadvantage of currently available silver alloys, which include substantial amounts of copper to maintain hardness, is their tendency to form fire scale and/or become tarnished. Such tarnish can occur from oxidation which results from exposure to air and/or sulphurization which results from exposure to sulfur from atmospheric contamination, or, for example, skin contact. The discoloration is usually treated with an appropriate polish to remove the discoloration. Such treatment must be performed regularly in order to maintain the beauty and condition of the article. The discoloration is a principal drawback to greater popularity of silver jewelry.
A number of silver alloys have been developed to attempt to minimize the amount of discoloration occurring on the surface of articles made from such alloys, while maintaining advantageous qualities of strength and the like. U.S. Pat. No. 3,811,876 to Harigaya et al. discloses a silver alloy including tin (Sn), zinc (Zn) and indium (In), with a balance being silver. The combination of Sn, Zn and In with the silver was found to have a synergistic effect in reducing sulphurization of the alloy. However, these alloys will lose their sulphurization resistance if contacted with conventional phosphoric flux containing polish used as deoxidizing agents.
U.S. Pat. No. 5,817,195 to Davitz discloses a silver alloy including nickel (Ni), metal silicate, Zinc (Zn), Copper (Cu), with a balance being silver. The alloy may also contain up to 0.5% Indium (In).
U.S. Pat. No. 5,021,214 to Sasaki et al. discloses a silver alloy including indium (In), aluminum (Al), and copper (Cu). The use of indium and aluminum is indicated as a substitute for conventional palladium (Pd) in providing xanthation resistance without the expense associated with the use of palladium.
U.S. Pat. No. 6,139,652 to Carrano discloses silver alloys. The alloys include silver and an added element which consists essentially of at least one oxide of aluminum, antimony, cadmium, silicon, titanium, and zinc. The oxide is effective to improve tarnish resistance of the alloy. By using certain processing techniques, it is possible to also increase the annealed hardness of the alloy.
U.S. Pat. No. 4,973,446 to Bernhard et al discloses silver alloys. The alloys consist essentially of silver, silicon, boron, zinc, copper, tin and indium. The alloy provides an improved porosity and grain size as well as reducing fire scale. However, this alloy includes copper in amounts that contribute to tarnish.
There is an acute need for silver alloys which maintain the advantage of strength and ease in manufacture while reducing the amount of corrosion and/or tarnish of articles made therefrom.